Method of photographic development to a predetermined value of contrast



I p 1942- L. PLANSKOY 2,296,048

METHOD OF PHOTOGRAPHIC DEVELOPMENT TO A PREDETERMINED VALUE OF CONTRAST Filedvllarch 17, 1939 4 Sheets-Sheet l .J Affy Leon'h lonsko' \NVENTOC mm I J 4 Sheets-Sheet 2 Sept. 15, 1942.

. METHOD OF PHOTOGRAPHIC DEVELOPIENT TO A I N VEL NTOE M K L.

Lean-H Plansko 5 M- 15, 1942- L. PLANSKOY 9 METHOD OF PHOTOGRAPHIC DEVELOPMENT TO A PREDETERMINED VALUE OF CONTRAST Filed larch 17, 1939 4 Sheets-Sheet 3 Fig.5

31- I/ Q5 a2 g s Leon+\ Plans K 5 INVENTQYC r sept- 1942- L. PLANSKOY 2,295,043

METHOD OF PHOTOGRAPHIC DEVELOPMENT To A PREDETERMINED VALUE OF CONTRAST Filed larch 17, 1939 4 Sheets-Sheet 4 r;- i m W4 76' n W "a c: 5 a 7 3 r r r r k r r r Leon Ploqfik \NVENT will apply Patented Sept. 15, 1542 METHOD OF PHOTOGRAPH! DE VEIDP MENT TO A PREDETERMINED VALUE OF CONTRAST Leonti Pianskoy, Paris, France, assignor of onehaii to Process Development Corporation, New

York, N. Y.

Application March 17', 1939, Serial No. 262,344 In Great Britain March 26, 1938 1 Claim.

The difference in density by whichthe Dho- A tographic emulsion represents the ratio of exposures to which it has been subjected increases with the time of development to a limiting value. The ratio of increase of density in respect of increase of the logarithm of exposure over the straight portion of the characteristic curve of the photographic emu sion is constant for a givenemulsion and set of development conditions and is termed gamma" 2 l I 105; E' -40g E,

It is important to be able to \stop the development as soon as a definite value of gammae has been reached in order to satistythe relation for correct reproduction where 'm is the development constant negative record, *1 that for the positive record and 1R the overall gamma governing the contrast of the complete reproduction cycle. The Equation 2 shows that the errors in the value go on'multiplying, which in all cases where the conformity oi the record is important, such as colour or sound reproduction, trickivork, duplication of the original negatives, etc., is a cause of undesirable distortion.

The determination of the correct time value for development to a precise value of "gamma" meets with considerable diiilculty because of the many variables influencing the final result. The

temperature-must be keptconstant and so must be the motion of the film in respect of the'developing solution, as well as the rate of stirrin of the latter. Oxydation phenomena, age or the developing solution, method of its preparation, constancy and purity of the commercially available constituent chemicals as well as their reaction with such products as may be used before the development (such as densensitizing dy s) all influence the time after which the gamma reaches the desired value. The time of development varies not only with diilerent emulsions but also with different batches of the same emulsion. The quantity of bromide liberated by the developing process depends am the image,characteristies, c. g. more bromide will be liberated when developing a negative image 0! a small black cross upon white backgroundthat when the pcsltive image is developed;the opposite when the cross is white and the background black. Since the bromide thus liberated actsas a-restr'ainer or development process, the

ditions or the Equation 1.

' operation. These difllculties-are well known action of'developer must be suitably prolonged. without any indication as to the magnitude of this effect during the development proper.

At present the development process is carried out during a time determined by photometrically 01' the .emulsion. When these densities are photometrically measured the value of "gamma" can be determined from the Equation 1. If the found value of gamma does not correspond with the desirable one, the-solution is replenished or else the time of development for the next length of film is varied. This method obviously only permits the control of developedfilm. Since the result of the control operation is known only after the operation to be controlled is completed,

any deviation of the value of gamma from the chose'noptimum in the measured sample is obviously repeated in the record as well. and to correct it further operations such as intensifying or reducing must be applied. These not only introduce additional lndeterminations in the final result but may also cause undesirable increase of graininess and/or distortion of the gradation of developing machine is employed, a further length of, film is processed during the time lapse between F the completion or the development of the control sample and its photometric reading, and this film-shows a further-deviation of gamma value from .thatdesired. In a unit development machine this deviation can be slightly reduced by keeping the machine idle during the d an several attempts have been madeto shorten the period between the completion 0! the develop-- ment and its photometric measurement. One 01 these attempts consists in the use 01 the photometers on the developing machine itself before.

the. completion of the drying process. But it is obvious that this solution reduces the delay between '-i.he end of the development and the measure only by the-timerequired for the drying process, still delaying the control during the period of fixing and washing; also the method the record. Moreover. when a continuous photometric while permitting the control the finished product does not allowto stop the processing automatically when the desired gamma is reached.

The present invention relates to a method of photographic development -to a predetermined value of contrast (gamma), which consists in preliminarily subjecting two areas of the film to be developed respectively to two exposures of different values, and observing through said two areas during the development process respectively two radiant beams, the intensities oi. which are so chosen that the difference between the logarithms of said intensities corresponds to the difference between the densities to be obtained in said areas after a correct develoment, and continuing said observation until the two radiant beams show no difference in accordance with Equation 6 on page 2.

By this method, it is thus possible to avoid all the above mentioned difiiculties encountered in the known methods, and to stop the development process at the exact moment when the desired value of gamma is reached.

In order to develop the film exactly to the predetermined gamma I proceed as follows. Two portions of the film are subjected respectively to two known control exposures so that log Ez-log E1=K (3) If the two exposures are made to be situated well within the straight portion of the characteristic curve of the film, when theyar'e made simultaneously and from a single light source, the slight variation of the film sensitivity or fluctuations of the source are 01' no importance,

receptors is made to close a relay thus indicating the completion of the development process.

The ballast density may consist of a filter, the density of which is equal to the difference between the densities to be obtained in the two portions which have been subjected to two diiterent exposures. Instead of a filter, use may be made of devices, such as a diaphragm, which reduce the section of passage of one 01' said beams. It is also possible to insert polarizing optical instruments, such as crossed Nicol prisms,

because it is the diiference and not the absolute .value of the exposure which is of importance in the Equation 3. The Equation 1 combined with 8 defines the relation and determines the difference of densities 121-121 which the control film must show when the development has been carried to the desired value of gamma. The film to be developed and carrying the control exposures is placed into the developing machine and subjected to the development process. Th control e posures pass during the-development process in the beams of a photometer, and preferably 01' a physical null method photometer, so that with increased development the intensity of the radiant energy incident on to the physical receptor or receptors decreases at the same time as the densities of the film increase. A ballast calibrated density equal to G or Dz-Di, is placed a into the beam passing through th control exthrough the other control exposure corresponding to the exposure value Es. When the ditierence of densities increases sufiiciently so that ZTs==Di+G the radiant energj from the two beams the condition or equilibrium in receptor or through which only a portion of the incident light passes in a, determined direction. Generally speaking, use may be made of any device adapted to lessen the intensity of one of said beams.

The physical photometric receptor must obviously be chosen to be sensitive to radiation to which the emulsion is not sensitive or has been desensitized, so that the beams of radiation passing through the control portions of the film cannot modify the exposure. In case of all normal non-desensitized emulsions a suitable infrared transmitting filter can beplaced over the source absorbing all radiation to which the emulsion is sensitive, the receptor being in this case a cesium or a cesium-cesium oxide-silver photoelectric cell, a selenium cell, a thalofide cell or any of the non-selective receptors such as thermopiles, bolometers, radiometers or their like. In case of desensitized films the radiant energy obviously depends upon the type oi desensitizer. For infra-red film a radiation of wave-lengths comprised between 500 m and 600 m and corresponding to the insensitive spectral region of the usual infra-red plates may be employed. In addition to the previously mentioned non-selective photometric receptors, cesium photoelectric cells and barrier E. M. F. photocells can be employed for. the measurement of the film densities within this region. a

One instrument advantageously used in connection-with the method is based upon the known null method of physical photometry in which two beams fall alternately and intermittently on to a photoelectric cell. As long as the intensities o! the beams are unequal, a pulsating current is led from the photocell into a galvanometer; when, however, the intensitis o! the two beams reach an equilibrium the current fiows continuously.

It is to be understood that the present invention does not relate to the measurement of the density or degree of blackening or the image, which depends both upon the exposure and the development; the invention relates to the control of the gamma, i. e. to the control of the degree of contrast of the image, which depends only upon the development. In the accompanying drawings, given by way of example:

Figs. 1 to 4 show diagrammatically four arrangements adapted to be used in the method 0! the present invention.

- Figs. 5 and 6 are respectively a sectional elevation anda plan view of a continuous developing machine modified according 'to the present invention.

Fig. 7 shows diagrammatically an electro-opgicaldrelay used with the arrangement 01 Figures an 6. a

Fig. 8 shows the electrical wiring diagram or the arrangements shown in Figs. 5 to 7.

Fig. 9 is a plan view oi a modification.

With reference to Fig. 1 a sourse S is placed pen'sable, to keep the film under continuous observation and consequently the control expobe located on the edge outside the perforation or 'is formed the image of the source. Since the lenses La and L4 are chosen of such local length as to place their focal plane in S: and 34 there issue from them two parallel beams which are reconverged by the lens Ls 'on to the photoelectric cell P producing on it an enlarged image of these two-apertures, superposed upon each other.

ror M. A disk G carrying a series of radial apertures is placed in close proximity of Si and S: and cuts the beams intermittently with a known frequency, kept constant by revolving the disk by means of a synchronous motor J. The optical units are enclosed in tubes T1 and T2, the extremities of which in the vicinity of the film haveburnished in vitreous silica or glass plates so as to prevent the access of the developing-solution into the optical system. Said tubes T1 and T: are inserted in apertures formed in the walls of the tank K containing the developing bath in such manner that they are liquid proof.

Y The current from the photoelectric tube or cell P passes into an audio-frequency amplifier indicated generally by the reference. A. As long as the development has not reached thedesired value of 'y, the beamsfalling 'onto the photoelectric cell are of different intensities and the pulsating current passes through the amplifier A onto the galvanometer relay R. When, however,

the gamma reaches the predetermined value, the Equation 6 is satisfied, the two beams become equal and the non-pulsating current issued from the cell cannot pass through the A. C. amplifier. The lack of current through the relay R causes a circuit to be closed which starts either a signal H or a motor which moves the film from the developer into a stop bath. The amplifier A' is equipped with frequency filters 'giving passage only to a narrow band corresponding to the frequenciesiwith which the "beams fall uponthe photo-cell. to minimize the pick-up and effects of stray light. l Whenit is desirable to use non-selective p ysical receptors or E. M. F. barrier cells, the sensitivity of which decreases considerably with insures should preferably be continuous. They can in the space reserved for sound recording in the picture record and vice-versa or else for use in unit developing machines they'can be located on a seperate length orthe sam emul'sion running in thesame bath over a separate set of rollers.

This latter course is not possible when it is desiredto use continuous developing machines. In

such continuous machines, the film I is guided as shown in Figures Sand 6, through a tank e containing a developing solution and over aseries of.

driving sprockets dito d1 in a form of festoons weighted by idle rollers 01 to as, and intoa stop bath-h. Assuming a constant driving speed, the development time for the portion of the film over the final sprocket in the developing tank is proportionate to the'sum of the lengths of all the iestoons. In consequence the length of the path of the film travel must at first be adjusted to satisfy the variables mentioned in the beginning of the present specification and then be constantly readjusted to compensate for such variations as can take place during the process of development.

Furthermore, both the initial adjustment as well as the continuous readjustment must be feasible.

over a considerable range. According to the present invention all these adjustments may be carried out automatically and are controlled by the previously described instruments. The sprockets ill to d-lvimmerged in the liquid of the bath are used to-drive the film I weighted idle rollers or to us. These rollers are guided vertically by rods ii to is and they can take any desired position in the solution between the levels .k1 and ks. Each of the rods ii to is carries two mercury switches 11 to 1:5 and 21 was which are tripped -by safety stops 7'1 to is when the idle rollers (11 to us reach the limiting .level In. The switches are mounted in opposition so that when anyone of the switches :m

It is possible with .this arrangement termittent radiation, the beams from L1 and L2 to a desired value of gamma balances the circult-thus closing the relay R. Alternatively differential E. M. F. barrier cells or bolometer or thermopile receptors N: and N4 can be. em-

ployed as shown by way' of example for a differential .bolometer in Figure 4. Obviously no disk is employed in these cases.

.It is of course preferable, although not indisto Is is open, the corresponding switch 21 to Z5 is closed. The measuring device 1 with :photome ter analogous to one ofthose shown in Figs. 1 to 4 'for measuring the densities of the two control exposures of the film is located between the.

sprockets ds and d1. It can be for example of the n is a motor revolving a series offrictionle'ss magnetic drives or clutches vi-m through the switches 31-15, and m is a motor revolving a series of frictionless magneticdrives or clutches ui-u-l through the switches 21-25. The potential applied to the drives ui-m through the switches zies' is lower than that applied to the drives 111-0: through the switches 11-13 and consequently the driving torque exerted by the former is smaller than that exerted by the latter. The motor n and the drives vi-va serve to feed 'the film when the conditions for the desired gamma are i'ulfilled, and the motor m and the drives ui-uv are used for adjusting the. lengths of the festoons when corrections are required 'in the conditions of I the development.

Fig. I shows an electro-optical relay usedin conjunction with the photometer l in which 91 is a-photoelectric cell receiving'that beam from the photometer l which goes throughthe lesser o! the two control exposures and the ballast density; p:

is a photoelectric cell receiving the beam going through the greater control exposure. a is a mirror galvanometer which projects an image-of a lensc illuminated by a-lamp s in the plane oi a barrier cell b, the current irom which closes the switch q (Fig. 8) through a secondary relay 1'. m and m: are prisms located in such manner that when 'the galvanometer beam is offset from its null position a portion 01' the image oi -c is deviated on to barrier cells bl or b: depending upon the direction oi the galvanometer deviation. The current from bl closes the circuit oi the motor nl through a secondary solenoid seli resetting relay rl causing the motor 111 to rotate in the same direction as the motor .n, while the current irom b: closes the circuit of the motor 4a through a secondary solenoid seli resetting relay n causing the motor m to rotate in a direction opposlte to that oi the motor 11. Before the beginning of development, the switches xl to :5 and o are closed, the switches zl to 25, q and t are open. and the machine is threaded with a leader him. the idle rollers having been located on the level It: so that the path oi the film in the machine is shorter than that required for development in a given solution ior a given film. It will be noted that as long as the switch t remains open, the relays fl and r: remain inoperative. The iilm is now spliced end to end with the leader and fed into the machine by starting in the usual manner the motor 11 which revolves the sprockets di-di through the magnetic drives 171-117, the circuit oi which is closed through the switches :1 to Is and 0. When the spliced end arrives at the sprocket d1, the thickness oi the splice lifts the magnetic roller switch -0 which cuts the current through beam is deviated through the prism m: on to the barrier cell In in consequence starting the motor nl through the relay r: in a direction oi rotation opposite to that'oi the motor 11. Those of the magnetic drives ill to in corresponding to the rollers al to as which are at the level kl and corresponding consequently also to the switches .21 to 25 which were closed during the preliminary adJustment oi the illm path through thedevelthe magnetic drives us and v3. whereby the the mercury switch 15 is tripped by the stop 15 so that the current is now out in the magnetic drive us. In turn the idle roller as leaves the level It: and the whole cycle of operation is periormed ior each oi the film loops until the dirlerence between two densities on the illm in the beams oi the photometer reaches a predetermined value corresponding to the desired gamma. This balances the photometer circuit and closes the switch a through the relay r, whereby all the magnetic drives 01 to m are now permanently energized. In consequence the film is now being driven along its new path through the develop.

ment tank 6 and into the stop bath h.

It will be seen that this new path is set automatically by the physical photometer, and that it takes into account all variables present at the beginning oi the development process, the compensation of which variables is required in order to develop the film to a desired value oi gamma. However it may be necessary to change this path due to exhaustion or iaulty replenishment of the solution or, to variation oi temperature during the subsequent stages or development. Any neccssity oi increasing or decreasing the time oi development is represented by a desequilibrium oi the photometer circuit and is compensated for through the electro-optical relay in the following manner. I

when the splice joining the leader to tlie film 'goes over the sprocket dc guiding it out from the stop bath )1, thesecond magnetic roller switch t is closed.

.When the film reaching the photometer is developed to a lower gamma than that ior which the instrument is in equilibrium the beam of the galvunometer is displaced anda portion'oi the oping machine rotate now in opposite direction to that oi the drives vl-v-l. Since the current supplied to. the drives in to 11.1 is of a lesser potential than that supplied to the drives vl-v-l, the two magnetic forces acting upon the sprockets are not in equilibrium and consequently said sprockets are not stopped. However the extra load which is now the duty of the mptor n repercutes by slowing up this motor, which should be chosen oi a-type, the speed of which is variable with the load, for instance a series motor. This slowing up prolongs the development time oi the 'illm until a new equilibrium is reached.

motor in in the same direction of rotation as that of the motor n, so that the magnetic drives Isl-m corresponding to the rollers at thelevel kl now rotate together withthe drives Di-U7 in the same direction. Obviously in this case the reverse oi what has happened when the motor nl revolved in the direction opposite to that oi the motor n takes place; the sprockets corresponding to the idle rollers which are at the level kl are now driven by the two motors together and in consequence the load on the motor 1: is reduced, so that the speed of said motor n is proportionately increased. The time oi development is thus shortened until a new equilibrium is reached.

Everyone oi the clutches vi to in may be driven by an independent motor 1m to 107, as shown in Figure 9, and the path of the film through the machine can then be reset every time when the original path has to be compensated through the relaysrl and rz. Since only those of the magnetic drives in to an which are energized through the switches zl to Z5 revolve in opposition or together with the drives in to in, only the motors driving their corresponding or opposite drives in to in have their load increased or decreased. In consequence the rest of the sprockets continue to revolve at their normal speed. But the switches zl to 2: supply current to only those oi thedrives m to m which control the sprockets corresponding to the idle rollers which have already reached the level kl. In consequence, when the drives in to m revolve in opposition to the drives in to m, that portion oi the iilm nearest the photometer is slowed up, but since the rest of the sprockets continueto deliver the film at the normal speed, the take-up is slower than the delivery, subsequent rollers al to as descend to the level kl consequently tripping subsequent s tches zl toss and slowing up their corresponding drives 91 to 121. It can be easily seen that in this case a new and longer path through the solution has been automatically determined, and this cycle oi operations can be repeated within the capacity oi the machine, which is chosen to correspond to the maximum limit oi the variations when on the contrary the motor 111 is made to revolve in the same direction as the motors m to 107, only those of the motors which drive the sprockets corresponding to the idle rollers which are already at the level k1 have their load reduced and in consequence revolve faster than the rest of the sprockets. The take-up being new more rapid than the delivery of one of the-rollers, that roller goesup thus cutting its corresponding switch 21-25 and de-energizing the drives ui-u-l to which it corresponds. It can be seen that in this way a new and shorter path is automatically set in the machine and that this path remains until the developing conditions undergo another change. v

Obviously the invention is not limited to the embodiments herein represented and described. Instead of speeding up or slowing down the sprockets d1 to d1 by means of the motor 111 and the clutches m to in, any other arrangement adapted to obtain the same result may be used; for instance the motors n or 101 to 101 can be shunt motors or motors with separate excitation, the excitation being controlled by the relays n and 1:.

Having now described my invention what I claim; as new and desire to secure by letters Paten An equipment for the photographic development of a film to a predetermined value of contrast (gamma) comprising a developing tank adapted to contain adeveloping solution, rotatable sprockets adapted to engage a film and to feed said film through said tank, driving means adapted to rotate said sprockets, auxiliary driving means adapted to be rotated in two opposite directions, frictionless magnetic drives inserted between said auxiliary driving means and said sprockets and adapted to transmit to said sprockets a driving torque in the direction of rotation of said auxiliary driving means so as to further or to oppose the action of said first main driving means, optical means adapted to project on two areas of said film while in the developing tank respectively two radiant beams, the difference their passage through said film areas and adapted to control said auxiliary driving means in order to speed up or slow down said driving means and said sprockets according as the value of gamma becomes higher or lower than said predetermined value.

LEON'I'I PLANSKOY. 

